: Finding proper size carb
Apr 25th, 00, 04:38 PM
Does anyone know of a site where I can plug in my engine configuration and get the proper size carb needed for such a combination? I'm running a edelbrock 750 on my 468 that can easily climb to 6500 rpm, wanting to know if this carb is choking my mill.
68 468 700R4, and here it is;
Apr 26th, 00, 12:40 AM
go to :www.mortec.com
they have a lot of info. check out the section on carbs, they have a formula to figure out what size you need, but i'd say your ok or you could even go with a 850cfm.
i have a 396 and i'm using a 780cfm.
Apr 26th, 00, 03:08 PM
Mark, using the CFM formula and your specs I get 880CFM. (468 X 6500)/3456 = 880.2 This number is only good if your heads can flow 100% and I think stock heads flow 80%. So 880 for aftermarket heads and 704 with stock heads according to the math. I don't think Edelbrock makes a carb bigger than what you already have, but I don't remember what heads you have.
Apr 26th, 00, 09:14 PM
Edelbrock now offers a 800cfm carb
Apr 27th, 00, 09:59 AM
I ran the #'s on your set up and came up with about a 748 cfm. Just my opinion but, with your cam and all it wouldnt hurt to go with just an rpm intake. Just my .02
Apr 27th, 00, 10:52 AM
I think I'm just going to change intakes first and then possibly do a carb change if absolutly necessary. I might try to use a k&n stubstack to try to gain a little more cfm. Right now I'm running a 3" element with drop base air cleaner, I'm thinking I might have to swith to a 4" element to gain proper clearence between the stubstack and the lid of the air cleaner. Does anyone have a stubstack on the edelbrock carb, and if so does it go anyhigher than the choke horn?
68 468 700R4, and here it is;
Apr 28th, 00, 09:53 PM
71 camairo, Just wondering how you arrived at 748 CFM?? 880 CFM @ 85% airflow??
Apr 30th, 00, 12:01 PM
HOLLEY CARBURETORS by HP books gives the following formula:
(CID / 2) X (RPM / 1728) X VOLUMETRIC EFFICIENCY = CFM
465 / 2 = 232.5
6500 / 1728 = 3.76
232.5 X 3.76 = 874 CFM AT 100% VE
806 X .85 = 742.9 CFM
This is from a V8 oriented book and should be a very good source.
The Holley web page shows the same formula as GearHead used. It's simpler.
I'd add that if the engine has ported heads and cam and headers the VE may be higher than .85%. Also if running fairly low gears and a light car, you could gain upper rpm horsepower by going to a larger carb for less restriction at top end.
The best thing would be to borrow a larger carb from someone and try it.
I ran an 800 double pumper on my 350 LTI Camaro with 4.56 gears. it went better in high gear when I backed off the throttle just a smidgen at about 5500 to 6500 rpm. I was running the second design LTI solid lifter cam.
I switched to a circle track (short 1/4 mile oval) cam with 2000 to 6500 design range. And got an increase in power and full acceleration at full throttle. I guess I got the VE high enough to where the large carb didn't hurt me anymore.
I tried a 750 DP carb on it before the cam change. I noticed a loss of throttle response in the lower gears. Top end was nice.
The 800 has the same size venturi bores as a 780. As you know the 780 came on the Z/28 with 302. The 780 uses the vaccum secondaries which makes it much more forgiving on over carburation, and thinking about it, I wonder how many of those Z/28 carbs ever open all the way on a street car! (whithout bogging)
There should be a way of measuring intake vaccum while driving at full throttle, and determining from that if the engine could stand a larger carb. That way the volumetric efficiency would be compensated for without having to determine what it is.
The older I get, the faster I was!
[This message has been edited by davidpozzi (edited 04-30-2000).]
Apr 30th, 00, 12:46 PM
Just to try to keep it all in context. An article I've read took a Demon Carb with removable venturis and did some testing from 650cfm to 1150 cfm. The differences were minimal from 650 to 850, with the 650 probably being more driveable and efficient at low rpm but the top numbers were within 3%. The only big difference happened with the 1150 and that was over 5500rpm.
May 1st, 00, 05:04 AM
Using a program that ive got I took into consinderation of the vol. efficiency of his intake, rpms, and cubes and thats what it spit out. Has allways worked for me. http://www.camaros.net/forum/biggrin.gif